In order to increase output and torque, but also in order to reduce the fuel consumption and exhaust emissions of internal combustion engines for road vehicles, cam shaft phase setters for varying the inlet or outlet control times have become widespread. Hydraulic phase setters which can be operated using engine oil in accordance with the principle of the hydraulic pivoting motor have established themselves in terms of high reliability and a good cost-benefit ratio. Hydraulically operable cam shaft phase setters in which a control valve for controlling the application of pressure to pressure chambers which serve to adjust the phase position, and an electromagnet which serves to operate the control valve, are arranged centrally on the rotational axis of the cam shaft have become widespread, not least from a point of view of cost. Since the installation space available is only limited, and due to cost pressure and the large channel cross-sections in the oil feed which are required for rapid adjustment, solutions in which the pressure oil to the phase setter, which rotates together with the cam shaft, is fed via a channel, provided in the cam shaft, to the control valve which is likewise arranged centrally have proven advantageous. The pressure oil is fed to the rotating cam shaft from the cylinder head, typically via one of the cam shaft bearings, preferably a pivot bearing of the cam shaft. The invention relates to phase setters of the described type in particular.
The control valve is favourably arranged and configured such that the characteristic curve of the valve is independent of the pressure of the oil. Otherwise, the setting of intermediate positions in the phase position of the cam shaft would for example be made more difficult or even prevented. It is therefore desirable if no resultant axial forces or only negligibly small resultant axial forces can be exerted on the valve piston of the control valve by the pressure oil, despite the changing pressure during operation of the internal combustion engine, so as not to disrupt the equilibrium of forces between the electromagnet acting on the valve piston and a valve spring which usually counteracts the electromagnet.
In order to avoid the resultant axial thrust, phase setters which rotate with the cam shaft and are supplied with the pressure oil via the cam shaft are usually supplied with the pressure oil via feeds which are directed towards the valve piston radially from without. Phase setters of this type are disclosed for example in DE 199 55 507 C2, DE 103 46 443 A1 and DE 196 54 926 C2. However, applying the pressure oil in this way entails channel guides which are expensive to produce, in particular P-type conduction. It is also difficult to configure the channels with large channel cross-sections, which are favourable for high adjusting speeds.
In order to avoid the problems described, cam shaft phase setters are known—for example from DE 198 48 706 A1 and DE 103 22 394 A1—in which the central control valve is arranged such that it cannot be rotated relative to an engine housing of the internal combustion engine, such that the cam shaft rotates relative to the control valve. The different oil feeds and oil drains to and from the control valve are separated from each other by means of shaft sealing rings, which however causes increased design expense and significant additional costs, resulting for instance in increased demands on the tolerances for the components which determine the radial position of the control valve relative to the cam shaft.
An additional problem known from the cited prior art is caused by arranging the coil of the electromagnet such that it is rotationally fixed relative to the engine housing of the internal combustion engine, as is preferred, while the anchor of the electromagnet is connected, rotationally fixed, to the valve piston of the control valve. The rotating anchor exhibits a practically unavoidable radial offset with respect to the coil, which causes radial forces which act on the anchor and thus on the valve piston and have to be absorbed by the tribological pairing of the valve housing and the valve piston. This in turn makes it more difficult to fulfil the requirement for a minimum possible hysteresis of the characteristic curve of the valve and increases the wear on the sliding areas of the tribological pairing.